1. Field of the Invention
The present invention relates generally to a magnetron for microwave ovens, and more particularly, to a magnetron for microwave ovens, which is provided with a filter box having a reduced height.
2. Description of the Related Art
In general, magnetrons are widely used to generate microwaves in home appliances, such as in microwave ovens, as well as in industrial applications, such as in high-frequency heating apparatuses, in particle accelerators and in radars.
The general construction of such a magnetron is described with reference to FIG. 1.
As shown in FIG. 1, in a magnetron 100, a plurality of vanes 102 that comprise a positive polar section together with a positive polar cylinder 101 are radially arranged at regular intervals in the positive polar cylinder 101 to form a cavity resonator, and an antenna 103 is connected to one of the vanes 102 to induce microwaves to an outside of the magnetron 100. Further, a filament 106 having a coil spring form is disposed along a central axis of the positive polar cylinder 101, and an activating space 107 is provided between radially inside ends of the plurality of vanes 102 and the filament 106. An upper shield 108 and a lower shield 109 are attached to a top and a bottom of the filament 106, respectively. A center lead 110 is fixedly welded to a bottom of the upper shield 108 while passing through a through hole of the lower shield 109 and the filament 106. A side lead 111 is welded to a bottom of the lower shield 109. The center lead 110 and the side lead 111 are electrically connected to first and second terminals 104a and 104b, respectively, and the first and second terminals 104a and 104b, respectively, being connected to an external power source (not shown). Thus, in the magnetron is formed an electrically closed circuit in which the first terminal 104a, the center lead 110, the upper shield 108, the filament 106, the lower shield 109, the side lead 111 and the second terminal 104b are electrically connected to each other in sequence. Other parts of the magnetron 100 except for parts comprising the electrically closed circuit are grounded. First ends of first and second choke coils 105a and 105b are electrically connected to the terminals 104a and 104b, respectively, while second ends of the first and second choke coils 105a and 105b are electrically connected to respective terminals of a capacitor (not shown), which is mounted on a side wall of a filter box 113 accommodating first ends of the center and the side leads 110 and 111, respectively, and the first and second terminals 104a and 104b or the first and second choke coils 105a and 105b. The filter box 113 is made of a metallic material to eliminate noise components irradiated through the center and side leads 110 and 111, and is grounded. Further, an upper permanent magnet 112a and a lower permanent magnet 112b are provided to apply magnetic flux to the activating space 107 with opposite magnetic poles of the upper and lower permanent magnets 112a and 112b facing each other. The positive polar section and the permanent magnets 112a and 112b are accommodated in and supported by a yoke 117. An upper pole piece 114a and a lower pole piece 114b are provided to induce rotating magnetic flux generated by the permanent magnets 112a and 112b into the activating space 107. An upper shield cup 115a and a lower shield cup 115b are tightly welded to the top of the upper pole piece 114a and the bottom of the lower pole piece 114b, respectively.
An insulating ceramic 116 is tightly and fixedly welded to a bottom of the lower shield cup 115b not only to seal an interior of the positive polar cylinder 101 in a vacuum state but also to prevent a dielectric breakdown phenomenon caused by a great potential difference between a bottom of the yoke 117 and the first and second terminals 104a and 104b or the first and second choke coils 105a and 105b. The center lead 110 and the side lead 111 are extended through holes formed in the insulating ceramic 116 to pass through the bottom of the yoke 117, and are connected to the first and second terminals 104a and 104b, respectively.
As shown in FIG. 2, when the magnetron 100 having the above-described construction is employed in a microwave oven 200, the magnetron 100 is disposed in a machine room 202 of the microwave oven 200 and irradiates microwaves into a cooking cavity 201 of the microwave oven 200. Parts including a high voltage transformer 204, a high voltage condenser (not shown) and a fan motor (not shown) are arranged under the magnetron 100. The insulating ceramic 116 is maintained to have a thickness of 16 mm or more so that a secure insulation distance is maintained between the bottom of the yoke 117 including the grounded lower shield cup 115b and the first and second terminals 104a and 104b or the first and second choke coils 105a and 105b so as to improve the efficiency of the magnetron 100 and to prevent harming the user. Further, a spaced distance of 15.5 mm or more must be maintained between the first and second terminals 104a and 104b or the first and second choke coils 105a and 105b and a bottom of the filter box 113 accommodating the first and second terminals 104a and 104b or the first and second choke coils 105a and 105b. Furthermore, a diameter of each of the first and second choke coils 105a and 105b must be taken into account. Accordingly, an entire height of the filter box 113 generally is about 43 mm or more. Further, a vibration and a noise may be generated between the filter box 113, which is magnetically connected to the upper and lower permanent magnets 112a and 112b by a magnetic force, and an outside wall of the machine room 202, so a certain distance “d” must be maintained between the bottom of the filter box 113 and the outside wall of the machine room 202 to prevent the vibration and the noise from being generated therebetween.
To maintain the above-described insulation distance and the certain distance “d”, an entire length of a magnetron must be increased and a width “D” of the machine room 202 must be widened in proportion to increases in a length of the magnetron. Accordingly, a ratio of a volume of the machine room 202 to a volume of the microwave oven 200 must be greater than that of a volume of the cooking cavity 201 to the volume of the microwave oven 200. For this reason, the conventional microwave oven is problematic in that a design of the microwave oven 200 is significantly restricted. In particular, this problem is fatal to small-sized microwave ovens having a cooking cavity of a small volume.